1. Field of the Invention
The present invention relates to a double-ended high-pressure discharge lamp and method of manufacturing it.
2. Description of the Related Art
In recent years, liquid crystal projectors have become well known for displaying enlarged projected images of letters and drawings, etc. Since such image projection devices require a prescribed optical output, high-pressure discharge lamps of high luminance are usually employed as the light source. Typically, such a lamp is combined with a reflecting mirror. Recently, in order to improve the convergence of the reflecting mirror, shortening of the arc length of the high-pressure discharge lamp has been demanded. However, such shortening of the arc length is associated with a drop in the lamp voltage, so if it is desired to operate the lamp with the same lamp power, lamp current must be increased. Increasing the lamp current leads to increased electrode loss and creates evaporation of the electrode material, resulting in early deterioration of the electrode, i.e. tends to shorten the life of the lamp. For these reasons, if the arc length is to be shortened, usually the mercury vapor pressure during lamp operation is increased, in order to avoid a drop in lamp voltage (increase in lamp current).
If the mercury vapor pressure during lamp operation is increased, it is necessary to construct the lamp in such a way that it will not break under this high operating pressure. A powerful means for preventing such lamp breakage is disclosed at page 111 of the Symposium Proceedings of The 7th International Symposium on the Science and Technology of Light Sources (1995). An outline of the details of this disclosure will be given using FIG. 7A and 7B.
FIG. 7A shows the construction of a prior art high-pressure discharge lamp 130. 100 is a practically spherical light-emitting section made of quartz glass and 101 are side tubes likewise made of quartz glass extending from the light-emitting section 100. 102 are tungsten electrodes, 103 are molybdenum foils, and 104 are molybdenum external leads. These elements constitute electrode assemblies 105, wherein the electrode 102 at one end of each molybdenum foil 103 projects into light-emitting section 100 and the other end of each molybdenum foil 103 is connected to one of the molybdenum external lead 104. The sealing of the discharge lamp in an air-tight manner is effected at the locations of the molybdenum foils 103 onto side tubes 101. Electrodes 102 each comprise a tungsten electrode rod 102a of diameter 0.9 mm and a tungsten coil 102b wound onto the electrode rod 102a in the vicinity of the end that projects into the light-emitting section 100. The external diameter L of the electrodes 102 with coils 102B would onto them is about 1.4 mm. A sealed-in material 120 comprising mercury or metal halide and argon gas (not shown) is sealed into the light-emitting section 100.
FIG. 7B is a cross-sectional view taken along a line VIIB--VIIB shown in FIG. 7A. Essentially, it is not possible to achieve perfect adhesion between the tungsten electrodes 102 and quartz glass, so a non-adhering part 107 is produced around each electrode 102. The width of this non-adhering part 107 is indicated by W. Such a cross-sectional view can be observed at any arbitrary cross-section in the range A-A' of FIG. 7A, i.e. from about the boundary of the light-emitting section 100 and the side tube 101 to the end of the molybdenum foil 103 (on the side where electrode 102 is connected).
In FIG. 7A, if the pressure within the light-emitting section 100 when the lamp 130 is operated is P (pressure P acts generally in the directions of the arrows 160 in the light-emitting section 100), as shown by arrows 170 in FIG. 7B, a pressure Pmax (&gt;P) larger than the pressure P generally indicated by the arrows 160 acts on this non-adhering part 107 (stress concentration phenomenon). Consequently, even if the pressure P within the light-emitting section 100 when the lamp 130 is operated is smaller than the breaking strength Plimit (considered to be about 400 atmospheres to 600 atmospheres, this breaking strength decreases if application of pressure is continued for a long time) of the glass that forms the light-emitting section, it is continued for a long time) of the glass that forms the light-emitting section, it is possible for a pressure exceeding the breaking strength of the glass to act at non-adhering part 107 (Pmax&gt;Plimit&gt;P). If this happens, the glass of the non-adhering part 107 breaks and lamp 130 is destroyed.
According to the disclosure, the magnitude of the pressure Pmax acting on non-adhering part 107 generally indicated by the arrows 170 due to stress concentration increases in proportion to the square root of the width W of non-adhering part 107 (Pmax.varies.P.times.W.sup.1/2). Consequently, if for example a pressure P of the same magnitude within light-emitting section 100 is considered, reducing the width W of non-adhering part 107 reduces the pressure Pmax acting on non-adhering part 107 and so increases the margin (Plimit-Pmax) with respect to the breakage strength Plimit of the glass, resulting in a lamp which is less likely to be destroyed (as described above, the breaking strength Plimit decreases if pressure continues to be applied to the glass for a long period, so some such margin is necessary for a lamp that is operated at high-pressure when operated to avoid being destroyed over a long period).
Conversely, if the width of the non-adhering part 107 is not changed, and the lamp 130 is operated with high-pressure P within light-emitting section 100, since the pressure Pmax acting on the non-adhering part 107 is large, the margin (Plimit-Pmax) with respect to the breaking strength Plimit of the glass becomes small, so the lamp can be easily destroyed.
From another point of view, considering the margin (Plimit-Pmax) with respect to a glass breaking strength Plimit of the same size, if the width W of the non-adhering part 107 is decreased, the pressure P within the light-emitting section 100 may be allowed to have correspondingly larger values. That is, lamp 130 can be operated (lit) with a higher pressure.
Due to the above, the extent to which stress concentration can be reduced by decreasing width W of this non-adhering part 107 is a vital point in preventing destruction when the lamp operating pressure is made high.
Conventionally, therefore lamps were manufactured in which the width W of the non-adhering part 107 was reduced by a method as disclosed in, for example, Early Japanese Patent Publication H. 7-262967 in order to prevent destruction of the lamp when this was operated with raised pressure in order to shorten the arc length. This prior art method of manufacture is described below.
FIGS. 8A, 8B, 8C and 8D are views given in illustration of an outline of the conventional method of manufacture of a high-pressure discharge lamp 130.
A prescribed light-emitting section 100 is formed by thermally expanding a quartz glass tube constituted by a glass bulb 110 in FIG. 8A manufactured in a separate process. Side tubes 101 are constituted by undeformed quartz glass attached to both ends of light-emitting section 100. While rotating this glass bulb 110 as shown by arrow 115 on a rotatable chuck, not shown, that grips both ends of side tubes 101, the boundary regions of light-emitting section 100 and side tubes 101 are heated by burners generally shown by arrow 111. Reduced-diameter sections 113 indicated by the shaded regions in which the internal diameter at that location is smaller, are formed by applying pressure to softened locations of side tubes 101 by means of freely rotating carbon heads 112.
After reduced-diameter sections 113 have been formed in the vicinity of both ends of light-emitting section 100 as described above, next, as shown in FIG. 8B, electrode assemblies 105 are inserted into side tubes 101 such that one end of electrode 102 constituting part of electrode assembly 105 is positioned within light-emitting section 100. Then, by heating the locations of molybdenum foil 103 to soften the glass sufficiently by means of burners generally indicated by arrows 121 over a suitable length from the vicinity of reduced-diameter section 113 (near the molybdenum foil 103) to external leads 104, the electrode assemblies 105 are sealed into the side tube 101 by clamping with a pair of clamping elements, not shown, or by compressing to a flattened shape. Molybdenum foil 103 having a thickness of about 20 microns expands, filling up the gap with the glass so that gas-tightness is maintained at the location of the molybdenum foil 103.
Next, as shown in FIG. 8C, material 120 to be sealed-in is inserted into light-emitting section 100 from side tubes 101 which are currently as yet unsealed and electrode assemblies 103 are then inserted into side tubes 101. In this condition, just as in FIG. 8B, the side tubes from reduced-diameter sections 113 to external leads 104 are softened by heating with burners, generally indicated by arrows 121, and the electrode assemblies 105 are sealed onto the side tube 101 by clamping with a pair of clamping elements, not shown, or by compressing to a flattened shape to complete the conventional high-pressure discharge lamp 130, shown in FIG. 8D, in the same way as in FIG. 7A.
FIG. 9 is a detailed view of the vicinity of the boundary (portion A of FIG. 7A or FIG. 8D) of light-emitting section 100 and side tube 101 of a conventional lamp 130. As described above, since essentially it is not possible to achieve perfect adhesion between the tungsten electrode 102 and quartz glass, a gap with respect to the glass is formed around the periphery of electrode 102 (non-adhering part 107 in FIG. 7B). As shown in FIG. 9, the width of this gap is not uniform, but in the case of a lamp manufactured by the conventional method of manufacture described above, the gap is largest in the vicinity of the boundary of light-emitting section 100 and side tube 101 and diminishes towards the molybdenum foil 103. The gap's greatest width is called Wmax. The greatest pressure (concentrated stress) Pmax (.varies.Wmax.sup.1/2) acts where this width is largest.
In the prior art method of manufacture disclosed in Early Japanese Patent Publication H. 7-262967 described above, electrode assemblies 105 are inserted from side tubes 101 after diameter reduction of the boundary region of light-emitting section 100 and side tube 101 to form the reduced-diameter sections 113 and one end of electrodes 102 must be positioned within the light-emitting section 100. Consequently, lamps can only be manufactured wherein the width Wmax of the gap (non-adhering part 107) in the vicinity of the boundary of light-emitting section 100 and side tube 101 is always larger (Wmax&gt;L) than the diameter L=1.4 mm (&gt;d) of the location where coil 102b is wound onto the electrode rod 102a of the greatest diameter on the side projecting into light-emitting section 100 of electrode 102, i.e. diameter d=0.9 mm. Consequently in a conventional high-pressure discharge lamp 130 there was the problem that, since a construction was adopted in which Wmax&gt;L, the pressure Pmax acting on non-adhering part 107 could not be made sufficiently small, making the lamp liable to fail.
To take a specific numerical example, in the case of a lamp 130 manufactured by the conventional method in which the electrode rod 102a was of diameter d=0.9 mm and the external diameter in the portion where the coil 102b was wound was L=1.4 mm, the maximum width Wmax of the gap between electrode 102 and the glass constituting side tube 111 was about 1.5 mm. In this case if a small hole is provided in light-emitting section 100 and the pressure within light-emitting section 100 is increased by feeding high-pressure gas in from this hole, destruction of lamp 130 is caused when the pressure of the high-pressure gas fed into light-emitting section 100 reaches about 120 atmospheres.
As to the lamp formed by electrode 102 having electrode rod 102a but having no coil 102b, an internal diameter rw of the reduced-diameter section 113 shown FIG. 8A, can only be reduced to d+.DELTA.d (d=diameter of electrode rod 102a). According to the present technology .DELTA.d is equal to 0.4 mm, but .DELTA.d can be as small as 0.1 mm. Theoretically the internal diameter rw can be made smaller than d+0.4 mm, such as to d+0.1 mm, but practically, from the view point of the present technology, the internal diameter rw is preferably d+0.4 mm as explained below.
When the internal diameter rw is made smaller than d+0.4 mm, a gap between the glass and the electrode 102 (electrode rod 102a) becomes so small that it will be very difficult to insert the electrode 102 (electrode rod 102a) through the reduced-diameter section 113, resulting in low productivity. Furthermore, when the internal diameter rw is made small, it will be very difficult to insert the material 120 in the light-emitting section 100. However, when the technology for inserting the electrode 102 (electrode rod 102a) as well as the material 120 is improved, the internal diameter rw can be made as small as d+0.1 mm.
It is an object of the present invention to solve the above problems and to provide a high-pressure discharge lamp of the double-ended type having a construction that is not liable to fail and a method of manufacturing the high-pressure discharge lamp.